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1. To establish fundamental understanding of heat transfer in electronic equipment. 2. To select a suitable cooling processes for electronic components and systems. 3. To increase the capabilities of post-graduate students in design and analysis of cooling of electronic packages. 4. To analysis the thermal failure for electronic components and define the solution. Course Goals

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Thermal convection Example 3.1: An electric current is passed through a wire 1mm diameter and 10 cm long. This wire is submerged in liquid water at atmospheric pressure, and the current is increased until the water boils. For this situation h = 5000 W/m2.oC. And the water will be 100 o C. How much electric power must be supplied to the wire to maintain the wire surface at 114 o C? Schematic: Electric wire Water

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Thermal convection Solution: The total convection loss from the wire is given by. For this problem the surface area of the wire is A= π d L = π (1 x 10-3) (10 x 10-2) = x10-4 m2 The heat transfer is therefore And this is equal to the electric power which must be applied.

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Thermal radiation The mechanism of heat transfer by radiation depends on the transfer of energy between surfaces by electromagnetic waves in the wave length interval between 0.1 to 100 μm. Radiation heat transfer can travel in vacuum such as solar energy. Radiation heat transfer depends on the surface properties such as colors, surface orientation and fourth power of the absolute temperature (T 4 ) of the surface. The basic equation for radiation heat transfer between two gray surfaces is given by

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Thermal radiation Example 3.2. A horizontal steel pipe having a diameter of 10 cm is maintained at a temperature of 60 o C in a large room where the air and wall temperature are at 20 o C with average heat transfer coefficient 6.5 W/m 2 K. The emissivity of the steel is 0.6 calculate the total heat lost from the pipe per unit length.

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Thermal radiation Solution: The total heat lost from the pipe due to convection and radiation Because the pipe in a large enclosure then the geometrical factor ƒ = 1

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There exists an analogy between the diffusion of heat and electrical charge. Just as an electrical resistance is associated with the conduc­tion of electricity, a thermal resistance may be associated with the conduction of heat. Analogy between Heat Transfer and Electric Circuits

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Series Circuits: By analogy

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Parallel Circuit:

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Combined Modes of Heat Transfer Combined Convection and Radiation

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Now if we define the arithmetic mean temperature as: If further T s -T e <

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Combined Convection and Conduction This combination is likely to occur with the use of extended surfaces where the primary surface exchanges heat by convection to the adjacent fluid flow and by conduction through the extended surfaces. This case may be considered in a similar manner as the above, but here the problem doesn't need extra work as the conduction thermal resistance is predefined.

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Overall Heat Transfer Coefficient Fluid combinationU, W/m 2.ºK. Water to water Water to oil Steam condenser, water in tube Ammonia condenser, water in tube Finned tube heat exchanger, water in tubes air in cross flow 25-50